This invention relates generally to hermetic refrigeration compressors of the reciprocating piston type, and more particularly to such compressors of the small displacement type used in household appliances.
Hermetic refrigeration compressors of the reciprocating piston type find widespread application in such household appliances as refrigerators. freezers, and small room air conditioners. For such reasons as low cost and small size, as well as electrical efficiency, such compressors are usually powered by two pole induction motors which, therefore, have a general operating speed of 3500 rpm, using a 60-hertz power supply. Because of this high speed of operation, it is highly desirable to use a relatively large diameter piston having a relatively short stroke to maintain low average piston velocities to minimize wear and improve mechanical efficiency. However, with a relatively large diameter cylinder bore, together with a short stroke, it tends to become difficult to minimize the clearance volume, which is defined as the space occupied by the compressed refrigerant within the cylinder bore and valve system when the piston is at the end of the compression stroke. The greater the clearance volume, the lower the compressor efficiency, and therefore many efforts are made to reduce the clearance volume to a minimum practical with the other requirements such as the compressor valving.
The valving of compressors of this type usually comprises a single reed valve for each of the suction or intake and discharge or exhaust functions of the compressor. These valves are normally in the form of a reed valve spring-biased to a closed position against a valve seat and openable during the appropriate portion of the compressor cycle by either the incoming or suction gases and during the compression portion of the piston cycle by the discharge gases. In a typical arrangement, the open end of the cylinder is closed off by a flat, parallel sided valve plate through which extend generally cylindrical suction and discharge ports. The suction valve, which operates under the relatively low pressure of the suction gases, is formed as a cutout portion of a sheet of spring material, and fits against the underside of the valve plate adjacent the piston. Since the suction port is sealed off on the piston side, the ported side does not contribute to any of the clear ance volume, which, in the case of the suction valve, is limited to the cutout portion of the valve sheet defining the reed valvo.
On the other hand, the discharge valve is mounted on the outer side of the valve plate and, because of the high pressure of the discharge gases, generally requires a more rugged structure to provide efficient operation and long life. Because this valve seals on the outer side of the valve plate, the entire area of the discharge valve port becomes part of the clearance volume and generally forms a substantial portion thereof.
Heretofore, discharge valves generally required a rigid stop member, as well as back-up valve springs, to limit the movement of the reed valve, and these members had to be mounted on the valve plate itself to provide a simple and low-cost structure. One such arrangement is shown in U.S. Pat. No. 4,352,377, which utilizes a reed valve, a back-up spring, and a heavy, rigid stop member, all of which are mounted over a pair of mounting posts secured to the valve plate itself. A retaining spring, also secured to these valve posts, operates to secure the other members in place, and to provide an easy arrangement for assembling the various parts of the discharge valve in place during the assembly of the compressor.
It has been recognized that on approach to improving the volumetric efficiency of the compressor by decreasing the clearance volume is to decrease the volume of the discharge port. To decrease this volume by decreasing the diameter of the opening has limited effectiveness because a decreased size opening results in greater restriction to flow of the discharged gases through this port, and is therefore self-defeating. The other alternative is to reduce the axial length of the discharge port, but reducing the overall thickness of the valve plate at all points has limitations because of the requirement for a minimum thickness for secure mounting of the posts as well as to resist the flexing of the valve plate under the alternating stresses applied during the suction and compressive strokes of the piston.
One approach to this problem has been by the use of a relatively thick valve plate having an elongated recess on the outer side into which the discharge port opens to thereby reduce its axial length. Since the valve plate at this point is too thin the mount support posts directly on the valve plate, this arrangement, as shown in U.S. Pat. No. 4,524,806, granted June 25, 1985, utilizes a frame member which fits within the recess and carries a pair of support posts at the opposite end. Thus, a reed valve, back-up spring, retainer plate, and retainer spring can then be mounted on the posts on this insert, which is made a frictional fit within the recess. While such an arrangement allows preassembly of all of the valve parts, it relies entirely upon the frictional fit of the insert within the recess to hold the valve assembly in place, so that any movement of the insert in the recess will tend to move the unit away from the valve seat, and thereby adversly affect the operation of the valve.